This invention relates generally to carbon nanotubes and nanofibers, and more particularly to methods for producing functionalized nanotubes, nanofibers, and nanoscale fiber films.
Carbon nanotubes and nanofibers have both rigidity and strength properties, such as high elasticity, large elastic strains, and fracture strain sustaining capabilities. Such a combination of properties is generally not present in conventional materials. In addition, carbon nanotubes and nanofibers are some of the strongest fibers currently known. For example, the Young's Modulus of single-walled carbon nanotubes can be about 1 TPa, which is about five times greater than that for steel (about 200 GPa), yet the density of the carbon nanotubes is about 1.2 g/cm3 to about 1.4 g/cm3. The tensile strength of single-walled carbon nanotubes is generally in the range of about 50 GPa to about 200 GPa. This tensile strength indicates that composite materials made of carbon nanotubes and/or nanofibers could likely be lighter and stronger as compared to current high-performance carbon fiber-based composites.
In addition to their exceptional mechanical properties, carbon nanotubes and nanofibers may provide either metallic or semiconductor characteristics based on the chiral structure of fullerene. Some carbon nanotubes and nanofibers also possess superior thermal and electrical properties such as thermal stability up to about 2800° C. in a vacuum and about 750° C. in air, thermal conductivity about twice as much as that of diamond, and an electric current transfer capacity about 1000 times greater than that of copper wire. Therefore, carbon nanotubes and nanofibers are regarded as one of the most promising reinforcement materials for the next generation of high-performance structural and multifunctional composites.
The use of nanoscale fibers in applications such as electronics, optics, thermal management, and high-performance composites have been hindered by technical roadblocks such as difficulties in their dispersion and their inert characteristics. Nanoscale fibers tend to form large bundles or big ropes, which may significantly limit potential applications. Conventional oxidization functionalization methods may etch the sidewalls, which may undesirably alter their mechanical properties. Effective functionalization to enhance dispersion, interfacial bonding, and functionality may be crucial to successfully transferring the exceptional properties of carbon nanotubes and nanofibers into many engineering applications.
Films of carbon nanotubes and nanofibers, or buckypapers, are a potentially important material platform for many applications. Typically, the films are thin, preformed sheets of well-controlled and dispersed porous networks of single-walled carbon nanotubes (SWNTs), multiple-walled carbon nanotubes (MWNTs), carbon nanofibers (CNFs), or mixtures thereof. The carbon nanotube and nanofiber film materials are flexible, light weight, and have mechanical, conductivity, and corrosion resistance properties desirable for numerous applications. The film form also makes nanoscale materials and their properties transferable to a macroscale material for ease of handling.
Epoxy polymers are easy to process and exhibit excellent mechanical properties with little toxicity. They are regarded as a standard matrix material for composites in various high-performance structural composite applications, where substantial strength, stiffness, durability, light weight, and good processability are required. For instance, epoxies have been used for years in the aerospace and boat industries. Carbon nanotube and/or nanofiber reinforced epoxy polymer composites are of interest in many applications; however, poor dispersion and weak interfacial bonding between carbon nanotubes and epoxy resin has hindered their widespread commercial adoption.
Fluorination may be used to modify nanoscale fiber functionality by additional reaction and yet have little effect on the mechanical properties of the nanoscale fibers. For example, fluorination can effectively enhance the functionality of nanoscale fiber films reinforced with polypropylene. However, fluorination is not always viable for carbon nanoscale fiber-epoxy film composites, for example, due to the negative effect of elemental fluorine on the epoxy curing reaction. Moreover, oxidization and fluorination functionalizations may result in very low yield rate and may involve long, multiple chemical reactions. Accordingly, scale-up and mass production using these approaches for functionalization may be difficult or cost ineffective.
It therefore would be desirable to provide functionalized nanoscale fibers and nanoscale fiber films which reduce or avoid the aforementioned deficiencies. In particular, it would be desirable to provide nanoscale fibers and nanoscale fiber films functionalized for composite applications. It also would be desirable to provide improved methods for functionalizing films for composite applications.